1. Cell:cell interactions have been examined using a novel flow cytometric technique. Two systems have been studied and compared: the FcGammaR-mediated aggregation of P388D1 cells with antibody-coated mouse spleen cells and the formation of conjugates between cloned CTL and splenic target cells. Many of the processes involved in the specific recognition and lysis of a target cell have been defined. 2. A survey of the expression of MHC class I molecules on mice of different strains has been made by measuring the binding of radiolabeled anti-class I monoclonal antibodies to mouse spleen cells. The study suggests that the levels of expression of class I molecules are strictly controlled, and vary by only small amounts between individual animals of the same or different strains. 3. Effector cells have been generated using heteroaggregates of anti-FcGammaR and anti-target cell antibodies. Unlike ADCC effector cells, these cells are not inhibited by immune complexes. These studies demonstrate that FcGammaR must be brought into close proximity to the target cell in order for lysis to occur. 4. The distribution of FcGammaR on mouse T-cells has been studied by dual parameter flow cytometry. At least 2 subsets of FcGammaR+ T cells have been identified, Lyt2+ and Lyt2-. The Lyt2+, FcGammaR+ subset increases in size in GMV-infected mice an may be responsible for suppression of allogeneic CTL responses in infected mice.